The present invention relates to a system and method of reproducing signals recorded on a magneto-optic recording medium, for reading information bits (magnetic domains) by magneto-optic effect and, more particularly, to techniques for enhancing track recording density and track density, and reproducing information recorded in a high density and for reproducing with high resolution.
According to the fundamental principle of a magneto-optic recording system, a portion of a magnetic thin film is heated locally to a temperature higher than the Curie temperature or the compensation temperature to nullify the coercive force of the heated portion and to invert the direction of magnetization of the heated portion in the direction of an external recording magnetic field applied thereto. Accordingly, the magneto-optic recording system employs a magneto-optic recording medium comprising a transparent substrate, such as a polycarbonate substrate, and a laminated recording layer formed on one major surface of the transparent substrate, and consisting of a magnetic recording film having an easy direction of magnetization perpendicular to its surface and having excellent magneto-optic characteristics, such as an amorphous rare earth metal-transition metal alloy film, a reflecting film and a dielectric film. The magneto-optic recording medium is irradiated with a laser beam from the side of the transparent substrate to read signals.
The track recording density of optical disks, such as digital audio disks (so-called compact disks) and video disks as well as a magneto-optic recording medium, is dependent principally on the SN ratio of reproduced signals, and the signal quantity of reproduced signals is greatly dependent on the period of the bit string of recorded signals, the wavelength of a laser beam emitted by the laser of a reproducing optical system, and the numerical aperture of the objective lens of the reproducing optical system.
The bit period f corresponding to a detection limit is expressed by: f=.lambda./2N.A., where .lambda. is the wave length of a laser beam emitted by the laser of the reproducing optical system, and N.A. is the numerical aperture of the objective lens.
Since crosstalk limiting the track density is dependent mainly on the intensity distribution (profile) of the laser beam on the surface of the recording medium, the track density, similarly to the bit period, is expressed generally by a function of .lambda./2 and the numerical aperture N.A.
Accordingly, the reproducing optical system employs, basically, a laser that emits a laser beam of a short wavelength .lambda. and an objective lens having a large numerical aperture N.A.
However, according to the present status of the art, an improvement in the wavelength .lambda. of the laser beam and the numerical aperture N.A. of the objective lens is limited. On the other hand, techniques have been developed to improve recording density through the improvement of the construction of the magneto-optic recording medium and the reading method.
For example, the applicant of the present patent application proposed a system that improves reproducing resolution by locally enlarging, reducing or extinguishing an information bit (magnetic domain) in reproducing a signal in Japanese Patent Laid-open (Kokai) Nos. Hei 1-143041 and Hei 1-143042, both incorporated herein. This system employs a magnetic recording layer of an exchange-coupled multilayer film consisting of a reproducing layer, an intermediate layer and a record hold layer, and reduces interference between information bits in reproducing signals by heating a magnetic domain of the reproducing layer with a reproducing light beam to enlarge, reduce or extinguish a portion of the magnetic domain heated at a high temperature to enable the reproduction of signals of a period beyond the limit of diffraction of light.
Although the track recording density can be improved to some extent by this system, it is difficult to improve the track density by this system.
Under such circumstances, the applicant of the present patent application proposed a novel signal reproducing method capable of preventing crosstalk and improving both track recording density and track density in Japanese Patent Laid-open (Kokai) No. Hei 1-229395, incorporated herein. This method employs a recording layer of a multilayer film consisting of a reproducing layer and a record hold layer magnetically coupled with the reproducing layer. The direction of magnetization of the reproducing layer is turned beforehand in a direction to erase signals in the reproducing layer. The reproducing layer is heated to a temperature higher than a predetermined temperature by irradiating the reproducing layer with a laser beam when reproducing signals so as to transfer only the magnetic signals written in the heated region of the record hold layer to the reproducing layer in order to read the magnetic signals.
In reading information recorded in information recording bits, i.e., bubble magnetic domains, formed in a magneto-optic recording medium by locally heating the magneto-optic recording medium with a laser beam by a magneto-optic recording/reproducing system utilizing magneto-optic mutual action, namely, the Kerr effect or Faraday effect, as explained above the recording bits must be formed in a reduced size to increase recording density for magneto-optic recording. However, the reduction in size of recording bits entails problems in resolution, as described above, in reproducing recorded information. Resolution is dependent on. the wavelength of the reproducing laser beam, and the numerical aperture N.A. of the objective lens.
A conventional magneto-optic recording/reproducing system will be described with reference to FIGS. 1A, 1B, 1C, and 1D. FIG. 1A is a typical top plan view of a recording pattern. A method of reproducing binary signals "1" and "0" recorded in recording bits 4, i.e. shaded portions in FIG. 1A, of a magneto-optic recording medium 3, such as a magneto-optic disk, will be described. A reading laser beam forms a circular spot 6 on the magneto-optic recording medium 3. When recording bits 4 are spaced so that the spot 6 is able to include only one recording bit 4 as shown in FIG. 1A, the spot 6 includes a recording bit 4 as shown in FIG. 1B, or the spot 6 does not include any recording bit 4 as shown in FIG. 1C. Accordingly, if the recording bits 4 are arranged at equal intervals, the output signal has, for example, a sinusoidal waveform whose amplitude varies alternately above and below a reference level 0 as shown in FIG. ID.
However, if recording bits 4 are arranged in a high density as shown in a typical plan view of a recording pattern in FIG. 2A, it is possible that the spot 6 includes a plurality of recording bits 4. Since a reproduced output signal provided when the two recording bits 4a and 4b among the successive three recording bits 4a, 4b, and 4c are included in one spot 6 as shown in FIG. 2B and a reproduced output signal provided when the two recording bits 4b and 4c are included in one spot 6 as shown in FIG. 2C are the same and cannot be discriminated from each other, the reproduced output signals form, for example, a straight line as shown in FIG. 2D.
Since the conventional magneto-optic recording/reproducing system reads directly the recording bits 4 recorded on the magneto-optic recording medium 3, the restrictions on the reproducing resolutions cause problems in S/N (C/N--carrier-to-noise ratio) and hence the magneto-optic recording/reproducing system is unable to achieve high-density recording and reproducing, even if the magneto-optic recording/reproducing system is capable of high-density recording, i.e., high-density bit formation.
The reproducing resolution dependent on the wavelength .lambda. of the laser beam and the numerical aperture N.A. of the lens must be improved to solve the problems in S/N (C/N). To solve these problems, the applicant of the present patent application proposed previously a magneto-optic recording/reproducing system capable of very high resolution (hereinafter referred to as "MSR system"), for example, in Japanese Patent Application No. Hei 1-225685, "Magneto-optic Recording/Reproducing Method", incorporated herein.
The MSR system enhances the reproducing resolution by reading only the recording bit 4 of a temperature in a predetermined temperature range on a magneto-optic recording medium by utilizing a temperature distribution formed by the relative movement between the magneto-optic recording medium and the spot 6 of the reproducing beam.
The MSR systems are classified into those of a so-called emergence type and those of an extinction type.
The MSR system of an emergence type will be described with reference to FIGS. 3A, 3B, 3C, and 3D. FIG. 3A is a typical top plan view of a recording pattern formed on a magneto-optic recording medium 10, and FIG. 3B is a typical sectional view showing a state of magnetization of the magneto-optic recording medium. As shown in FIG. 3A, the magneto-optic recording medium 10 moves in the direction of an arrow D relative to the spot 6 of a laser beam. As shown in FIG. 3B, the magneto-optic recording medium 10 is, for example, a magneto-optic disk having at least a reproducing layer 11 and a recording layer 13 formed of perpendicularly magnetizable films. The reproducing layer 11, the recording layer 13 and an intermediate layer 12 formed between the reproducing layer 11 and the recording layer 13 are provided. Arrows in the layers 11, 12, and 13 in FIG. 3B indicate the directions of magnetic moment. In FIG. 3B, magnetic domains indicated by downward arrows are in an initial state. Information recording bits 4 are formed at least in the recording layer 13 with magnetic domains magnetized upward for binary values "1" or "0".
In reproducing recorded information signals from the magneto-optic recording medium 10, an external initializing magnetic field H.sub.1 is applied to the magneto-optic recording medium 10 to magnetize the reproducing layer 11 downward, as viewed in FIG. 3B, for initialization. Although the recording bits of the reproducing layer are extinguished by initialization, the respective directions of magnetization of regions in the reproducing layer 11 and the recording layer 13 corresponding to the recording bits 4 are maintained reverse to each other by magnetic domain walls formed in the intermediate layer 12, so that the recording bits 4 remain in latent recording bits 41.
A reproducing magnetic field H.sub.r of a direction reverse to that of the initializing magnetic field H.sub.i is applied at least to the reproducing regions of the magneto-optic recording medium 10. As the magneto-optic recording medium 10 moves, the region having the initialized latent recording bit 41 comes under the spot 6. Since the duration of irradiation with the beam in the front side, the left side in FIGS. 3A and 3B, on the magneto-optic recording medium 10 with respect to the direction of movement is longer, a high-temperature region 14 is formed in the front side of the spot 6 as indicated by a shaped area enclosed by a broken line a. In the high-temperature region 14, magnetic domain walls in the intermediate layer 12 disappear, and the magnetization of the recording layer 13 is transferred to the reproducing layer 11 by exchange force, so that the latent recording bit 41 in the recording layer 13 emerges in the reproducing layer 11 in a reproducible recording bit 4.
Accordingly, the recording bit 4 can be read out by detecting the rotation of the plane of polarization of the spot 6 by magneto-optic effect, namely, Kerr effect or Faraday effect, corresponding to the direction of magnetization of the recording layer 11. Latent recording bits 41 in a low-temperature region 15, other than the high-temperature region 14, in the spot 6 do not emerge into the reproducing layer 11, and hence the reproducible recording bit 4 is included only in the narrow high-temperature region 14. Therefore, even if information is recorded in a high recording density on the magneto-optic recording medium 10 capable of high-density recording, in which a plurality of recording bits 4 are included in the spot 6, only one of the recording bits 4 can be read for high-resolution signal reproducing.
To carry out signal reproducing in such a mode, the initializing magnetic field H.sub.i, the reproducing magnetic field H.sub.r, the respective coercive force, values of thickness, intensities of magnetization and values of domain wall energy of the magnetic layers are determined selectively according to the temperature of the high-temperature region 14 and that of the low-temperature region 15. The coercive force H.sub.c1, thickness h.sub.1 and saturation magnetization M.sub.s1 of the reproducing layer 11, and the coercive force H.sub.c3, thickness h.sub.3 and saturation magnetization M.sub.s3 of the recording layer 13 must meet an expression in Mathematical 1 to initialize only the reproducing layer 11.
(Mathematical 1) EQU H.sub.i &gt;H.sub.c1 .sigma..sub.w2 /2M.sub.s1 .multidot.h.sub.1
where .sigma..sub.w2 is domain wall energy of the magnetic domain wall between the reproducing layer 11 and the recording layer 13.
An expression in Mathematical 3 must be met to maintain the information recorded in the recording layer 13 by the magnetic field.
(Mathematical 3) EQU H.sub.i &lt;H.sub.c3 -.sigma..sub.w2 /2M.sub.s3 .multidot.h.sub.3
An expression in Mathematical 4 must be met to maintain the magnetic domain walls formed in the intermediate layer 12 between the reproducing layer 11 and the recording layer 13 after the initializing magnetic field H.sub.i has been applied to the magneto-optic recording medium.
(Mathematical 4) EQU H.sub.c1 &gt;.sigma..sub.w2 /2M.sub.s1 .multidot.h.sub.1
An expression in Mathematical 5 must be met to heat the high-temperature region 14 at a selected temperature T.sub.H.
(Mathematical 5) EQU H.sub.c1 -.sigma..sub.w2 /2M.sub.s1 .multidot.h.sub.1 &lt;H.sub.r &lt;H.sub.c1 +.sigma..sub.w2 /2M.sub.s1 .multidot.h.sub.1
The magnetization of the latent recording bits 41 of the recording layer 13 can be transferred to, namely, binary values "1" and "0", which can be made to emerge in only regions of the reproducing layer 11 corresponding to the magnetic domain walls of the intermediate layer 12 by applying the reproducing magnetic field H.sub.r meeting the expression in Mathematical 5.
Although the magneto-optic recording medium 10 employed by the MSR system has the reproducing layer 11, the intermediate layer 12 and the recording layer 13 forming a three-layer construction, the MSR system may employ a four-layer magneto-optic recording medium additionally provided with an auxiliary reproducing layer 17 between the reproducing layer 11 and the intermediate layer 12 as shown in an enlarged schematic sectional view in FIG. 4.
The auxiliary reproducing layer 17 supplements the characteristics of the reproducing layer 11 to compensate the coercive force of the reproducing layer 11 at a room temperature to stabilize the magnetization of the reproducing layer 11 caused by the initializing magnetic field H.sub.i regardless of the existence of magnetic domain walls and to decrease the coercive force sharply at a temperature near the reproducing temperature so that the magnetic domain walls of the intermediate layer 12 expand into the auxiliary reproducing layer 17 to finally invert the reproducing layer 11 and to extinguish the magnetic domain walls for satisfactory emergence of the recording bits.
The coercive force H.sub.c1 of the reproducing layer 11 of a four-layer magneto-optic recording medium provided with the auxiliary reproducing layer 17 is substituted by H.sub.CA as expressed by an expression in Mathematical 6, and .sigma..sub.w2 /M.sub.s1 .multidot.h.sub.1 is substituted by .sigma..sub.w2 /(M.sub.s1 .multidot.h.sub.1 +M.sub.ss .multidot.h.sub.s).
(Mathematical 6) EQU H.sub.CA =(M.sub.s1 .multidot.h.sub.1 .multidot.H.sub.c1 +M.sub.ss .multidot.h.sub.s .multidot.H.sub.cs)/(M.sub.s1 .multidot.h.sub.1 +M.sub.ss .multidot.h.sub.s).
where H.sub.c1 &lt;H.sub.CA &lt;H.sub.cs for the MSR system of an emergence type.
In Mathematical 6, M.sub.ss h.sub.s and H.sub.cs are the saturation magnetization, thickness and coercive force, respectively, of the auxiliary reproducing layer 17.
The MSR system of an extinction type will be described hereinafter with reference to FIGS. 5A and 5B. FIG. 5A is a typical top plan view of a recording pattern formed on a magneto-optic recording medium 10, and FIG. 5B is a typical sectional view showing a state of magnetization, in which parts like or corresponding to those shown in FIGS. 3A and 3B are denoted by the same reference characters and the description thereof will be omitted to avoid duplication. This magneto-optic recording medium does not need any initializing magnetic field H.sub.i.
A reproducing operation for reproducing information recorded on the magneto-optic recording medium 10 will be described. The high-temperature region 14 is heated so that an expression in Mathematical 7 is satisfied, and then, an external reproducing magnetic field H.sub.r is applied to the magneto-optic recording medium 10 to extinguish recording bits 4 in the high-temperature region 14 included in the spot 6 of a laser beam in the reproducing layer 11 magnetized downward as viewed in FIG. 5B. Thus, the MSR system of an extinction type enables information recorded in only the recording bits 4 in the low-temperature region 15 in the spot 6 to be reproduced to improve the resolution.
(Mathematical 7) EQU H.sub.r &gt;H.sub.c1 +.sigma..sub.w2 /wM.sub.s1 .multidot.h.sub.1
However, the conditions including the coercive force are determined so that the recording bits 4 of the recording layer 13 remain in latent recording bits 41 in an extinction state to hold the magnetization of the recording layer 13, i.e., the recording bits 4, are transferred to the reproducing layer 11 and held therein in a reproducible state at a room temperature.
The foregoing MSR systems of an emergence type and an extinction type reproduce the recording bit in a local region included in the spot of the recording laser beam to reproduce the information in and enhanced resolution.
In reproducing signals by these previously proposed signal reproducing methods, however, the area to be transferred to the reproducing layer (reproducible area) expands with the increase of reproducing power, which deteriorates frequency characteristics of reproduction.